Gas turbine engines, such as those used to provide thrust for an aircraft, typically include a fan section, a compressor section, combustors, and turbines positioned sequentially in an upstream to downstream arrangement. In operation, air may be drawn into the engine, accelerated by the fan section, and then pressurized in the compressor section. After passing through the compressor section, the air may be mixed with fuel and combusted in the combustors to generate hot combustion gases. The hot combustion gases may then expand through and drive the turbines which may, in turn, drive the compressor section and the fan section by driving the rotation of an interconnecting shaft. After passing through the turbines, the air may be exhausted through an exhaust nozzle to provide some of the propulsive thrust to an associated aircraft or to provide power if used in land-based operations.
The fan section, the compressor section, and the turbines of a gas turbine engine may each include a plurality of airfoils which may be rotating blades or non-rotating stator vanes. The airfoils may be involved in altering the pressure, velocity, or direction of the air or gas flow. The airfoils typically include a root portion that is received by a support structure such as a rotor or a hub. For example, the support structure may have a slot with a shape that is complementary to the root portion of the airfoil.
Airfoils formed from nonmetallic materials, such as ceramic or ceramic matrix composite materials, may be advantageous for gas turbine engine applications because they may be relatively lightweight, high in strength, and/or thermally resistant. While nonmetallic airfoils are effective for these reasons, it may be challenging in some cases to provide a robust connection between the root portion of the nonmetallic airfoil and a corresponding slot of a support structure that is formed from a metallic material. In particular, mismatches in the coefficients of thermal expansion (CTE) of the nonmetallic material of the airfoil and the metallic material of the support structure may cause significant thermal strain at the connection interface upon exposure to large temperature differences or repeated thermal cycling.
An attempt to mitigate contact damage stress between nonmetallic airfoil roots and metallic support structures has been described in U.S. Pat. No. 6,132,175. This system utilizes a multi-layer compliant sleeve that slideably engages a ceramic airfoil root and rests between the ceramic airfoil root and a metallic support slot during operation to prevent airfoil fracture. However, additional enhancements that may provide improved performance capabilities are still wanting.
Clearly, there is a need for strategies that provide a robust connection between nonmetallic airfoils and metallic support structures having thermal expansion mismatches.